mkswap: improve randomness of UUID generation set version/variant bits
function old new delta
mkswap_main 317 410 +93
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/mkswap.c b/util-linux/mkswap.c
index 167b9ee..c4e30fd 100644
--- a/util-linux/mkswap.c
+++ b/util-linux/mkswap.c
@@ -53,14 +53,65 @@
#if ENABLE_DESKTOP
static void mkswap_generate_uuid(uint8_t *buf)
{
+ pid_t pid;
unsigned i;
char uuid_string[32];
- /* rand() is guaranteed to generate at least [0, 2^15) range,
+ /* http://www.ietf.org/rfc/rfc4122.txt
+ * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * | time_low |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * | time_mid | time_hi_and_version |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |clk_seq__and_variant | node (0-1) |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * | node (2-5) |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * IOW, uuid has this layout:
+ * uint32_t time_low (big endian)
+ * uint16_t time_mid (big endian)
+ * uint16_t time_hi_and_version (big endian)
+ * version is a 4-bit field:
+ * 1 Time-based version
+ * 2 DCE Security version, with embedded POSIX UIDs
+ * 3 Name-based version (MD5)
+ * 4 Randomly generated version
+ * 5 Name-based version (SHA-1)
+ * uint16_t clk_seq_and_variant (big endian)
+ * variant is a 3-bit field:
+ * 0xx Reserved, NCS backward compatibility
+ * 10x The variant specified in rfc4122
+ * 110 Reserved, Microsoft backward compatibility
+ * 111 Reserved for future definition
+ * uint8_t node[6]
+ *
+ * For version 4, these bits are set/cleared:
+ * time_hi_and_version & 0x0fff | 0x4000
+ * clk_seq_and_variant & 0x3fff | 0x8000
+ */
+
+ i = open("/dev/urandom", O_RDONLY);
+ if (i >= 0) {
+ read(i, buf, 16);
+ close(i);
+ }
+ /* Paranoia. /dev/urandom may be missing.
+ * rand() is guaranteed to generate at least [0, 2^15) range,
* but lowest bits in some libc are not so "random". */
- srand((unsigned)monotonic_us() + getpid());
- for (i = 0; i < 16; i++)
- buf[i] = rand() >> 5;
+ srand(monotonic_us());
+ pid = getpid();
+ while (1) {
+ for (i = 0; i < 16; i++)
+ buf[i] ^= rand() >> 5;
+ if (pid == 0)
+ break;
+ srand(pid);
+ pid = 0;
+ }
+
+ buf[4 + 2 ] = (buf[4 + 2 ] & 0x0f) | 0x40; /* time_hi_and_version */
+ buf[4 + 2 + 2] = (buf[4 + 2 + 2] & 0x3f) | 0x80; /* clk_seq_and_variant */
bin2hex(uuid_string, (void*) buf, 16);
/* f.e. UUID=dfd9c173-be52-4d27-99a5-c34c6c2ff55f */